1. Technical Field of the Invention
The present invention relates to a device and method for adjusting collision timing between an electron beam and laser light when an X-ray is generated by inverse Compton scattering.
2. Description of the Related Art
As means for generating an X-ray by a small-sized device, means capable of obtaining a quasi-monochromatic X-ray arisen from inverse Compton scattering by a collision between an electron beam and laser light is known (e.g., Non-Patent Document 1 and Patent Documents 1 to 3).
In “Small-Sized X-Ray Generator” of Non-Patent Document 1, as illustrated in FIG. 1, an electron beam 52 accelerated by a small-sized accelerator 51 (an X-band acceleration tube) is allowed to collide with laser 53 to generate an X-ray 54. The electron beam 52 generated by an RF (Radio Frequency) electron gun 55 (a thermal RF gun) is accelerated by the X-band acceleration tube 51, and collides with the pulse laser light 53. The hard X-ray 54 having a time width of 10 ns is generated by Compton scattering.
In this figure, reference numeral 41 denotes a power source, 42 denotes an α-magnet, 43 denotes a magnet, 44 denotes Q-magnets, 45 denotes a beam dump, 46 denotes a laser unit, 47 denotes a mirror, 48 denotes a lens, 49 denotes a laser dump, 50 denotes a synchronizer, and A denotes a collision point.
This device is miniaturized by using, as an RF, an X-band (11.424 GHz) corresponding to a frequency four times as high as that of an S-band (2.856 GHz) for general use in a linear accelerator, and it is predicted that the hard X-ray having, for example, an X-ray intensity (a photon number) of about 1×109 photons/s and a pulse width of about 10 ps will be generated.
“Method and Apparatus for Producing High Brightness X-Rays or γ-Rays” of Patent Document 1 have an object to accumulate laser light in an optical resonator having ultra-high reflectivity mirrors and use this light so that powerful high brightness X-rays or γ-rays are produced from even small initial laser.
Therefore, in this invention, as illustrated in FIG. 2, laser light from a laser 61 is injected into an optical resonator 62 and accumulated therein. The optical resonator 62 has ultra-high reflectivity mirrors 63, 64 having a mirror reflectivity of 0.999% or more. An electron beam may be introduced obliquely into the optical resonator 62 to make a collision. In the interaction area, X-rays or γ-rays 66 are produced due to Compton scattering. In this figure, reference numeral 65 denotes an accelerator.
“System and Method for X-Ray Generation” of Patent Document 2 have an object to generate X-rays via the process of inverse Compton scattering.
Therefore, as illustrated in FIG. 3, the system of this invention includes a high repetition rate laser 72 adapted to direct high-energy optical pulses 73 in a first direction 71 within a laser cavity 70 and a source 74 of a pulsed electron beam 78 adapted to direct the electron beam 78 in a second direction 76 opposite the first direction within the laser cavity 70. The electron beam 78 interacts with photons in the optical pulses 73 within the laser cavity 70 to produce X-rays 75 in the second direction 76. In this figure, reference numeral 79 denotes a pump laser.
“Multi-Color X-Ray Generator” of Patent Document 3 has an object to successively switch and generate a plurality of (two, three or more types of) monochromatic hard X-rays at short time intervals to such an extent that it may be judged that a blood vessel does not move, and generate an intense X-ray applicable to angiography or the like.
Therefore, as illustrated in FIG. 4, the device of this invention includes an electron beam generator 85 which accelerates an electron beam to generate a pulse electron beam 81 and which passes the beam through a predetermined rectilinear orbit 82, a composite laser generator 86 which successively generates a plurality of pulse laser lights 83a, 83b having different wavelengths, and a laser light introduction device 87 which introduces the plurality of pulse laser lights into the rectilinear orbit 82 to be opposed to the pulse electron beam 81, so that the plurality of pulse laser lights 83a, 83b successively head-on collides with the pulse electron beam 81 in the rectilinear orbit 82 so as to generate two or more types of monochromatic hard X-rays 84 (84a, 84b). In this figure, reference numeral 88 denotes a pump laser.
[Non-Patent Document 1]
“Development of Small-Sized Hard X-Ray Source using X-band Linac”, 27-th Linac Technology Research Meeting, 2002, authored by Katsuhiro DOHASHI, et al.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 7-110400 titled “Method and Apparatus for Producing High Brightness X-Rays or γ-Rays”
[Patent Document 2]
Japanese Patent Application Laid-Open No. 2005-285764 titled “System and Method for X-Ray Generation”
[Patent Document 3]
Japanese Patent Application Laid-Open No. 2006-318746 titled “Multi-Color X-Ray Generator”
As described above, there have been proposed various types of means for colliding laser light with an electron beam and generating an X-ray by inverse Compton scattering. In these conventional examples, as schematically illustrated in FIG. 9, it is assumed that a signal is sent from a synchronizer a (a master oscillator) to a high-frequency generator c and a laser unit d using a delay circuit b or the like at the proper timing and an electron beam is allowed to collide with laser light at a desired place.
FIG. 10 is a schematic diagram of a collision-timing adjusting method by a conventional synchronizer. In this figure, the horizontal axis represents the time, tR represents a time (hereinafter, referred to as “high-frequency delay time”) from a high-frequency generation moment to a moment when an electron beam reaches a collision point, te represents a time (hereinafter, referred to as “electron delay time”) from an electron generation moment to the moment when the electron beam reaches the collision point, and tL, represents a time (hereinafter, referred to as “laser delay time”) from a laser oscillation moment to the moment when the laser light reaches the collision point.
The above-described conventional method calculates in advance the high-frequency delay time tR, the electron delay time te, and the laser delay time tL from the device constitution, and presets a time dte (=tR−te: electron generation delay time) from the high-frequency generation moment to the electron generation moment and a time dtL (=tR−tL: laser generation delay time) from the high-frequency generation moment to the laser oscillation moment in each delay circuit b.
However, a time until the high-frequency generator c actually generates a high frequency HF after receiving a high frequency generation signal and a time until an electron generator (e.g., electron gun) actually generates an electron E after receiving an electron generation signal are not 0 in a narrow sense. According to a state of the high-frequency generator c or the electron generator (electron gun), the real generation timing may fluctuate (change). An electron immediately after generation is before acceleration by an acceleration tube, and is slightly slower than the light speed (e.g., about 90% of the speed of light).
Therefore, the above-described conventional method has a problem in that a real collision position (a real collision point) are different from a predicted collision point since times when the electron beam and the laser light reach the collision point are slightly different. As a result, an amount of X-rays generated is reduced since a collision area is reduced. On the other hand, a virtual focus (generation point) of an X-ray changes and an image captured using the focus is blurred.
FIG. 11 is a diagram schematically illustrating a collision situation between an electron beam and laser light. In this figure, reference numeral 1 denotes an electron beam, 3 denotes laser light, 4 denotes an X-ray, 8 denotes an allowed collision area, 9a denotes a predicted collision point, and 9b denotes a real collision point.
The predicted collision point 9a is preset on a common orbit (optical path) of the laser light 3 and the electron beam 1. The laser light 3 is pulse laser light incident from the left to the right in this example, and is concentrated at the predicted collision point 9a to have a minimum light-focusing diameter (e.g., 1 μm or less)
The electron beam 1 is an electron beam bunch incident from the right to the left in this example. When the electron beam 1 reaches the predicted collision point 9a along with the laser light 3, a collision rate between the two is maximized and a maximum amount of X-rays 4 is generated. Since the collision rate is sufficiently high before and after the predicted collision point 9a, for example, a range where a light-focusing area is equal to or less than twice the predicted collision point 9a is regarded as the allowed collision area 8. The allowed collision area 8 has, for example, a range of several 10 mm before and after the predicted collision point 9a. 
The speed upon collision of the electron beam may reach substantially the light speed (about 300,000 km/s=3×108 m/s in a vacuum). Therefore, even when a time in which the electron beam 1 reaches the predicted collision point 9a is only 1 ns (=10−9 s) later than that of the laser light 3, a difference between the real collision point 9b and the predicted collision point 9a is ΔL (=about 300 mm). Since a deviation from the allowed collision area 8 is also large and the pulse laser light 3 is greatly spread out as compared with the minimum light-focusing diameter, the collision rate is very lowered (substantially close to 0) and the above-described problem occurs.
The present invention has been made to solve the above-described problem. That is, an object of the present invention is to provide a device and method for adjusting collision timing between an electron beam and laser light, which may precisely position a real collision point between the electron beam and the laser light at a predicted collision point or the neighborhood thereof even when the timing of generating an electron or an electron beam fluctuates (changes), thereby increasing a collision rate between the two to increase an X-ray generation output and preventing a virtual focus (generation point) of an X-ray from being changed to increase a resolution of an image captured using the X-ray.